The following references describe relevant background art in that they disclose control circuits for controlling battery-powered inverters used for AC power supply backup.
U.S. Pat. No. 4,508,974 to Henderson discloses a logic controlled battery-powered inverter for use with a motor generator set that is used to provide clean, noise-free power to a load such as a computer. The inverter operates only when normal utility power fails, and employs the ability of the motor generator set to handle brief power outages to power the load for the brief time needed to decouple the main line power supply and couple the inverter to the motor generator.
U.S. Pat. No. 4,667,116 to Honjo et al. discloses an invertor control circuit for switching a load from an inverter to a commercial power source uninterruptedly by means of a thyristor when access to the synchronization circuit of the inverter is required for repair or servicing.
U.S. Pat. No. 4,946,096 to Ballard et al. discloses a method and apparatus for operating a furnace from a 12 V DC battery. A furnace which normally operates from a residential AC power source is adapted to operate on auxiliary DC power by changing the normal control function to prevent the furnace from operating in the high heat mode during periods of auxiliary power use.
U.S. Pat. Nos. 4,956,563 and 5,055,703 to Schornack, the Schornack '563 and the Schornack '703 references, disclose a battery operated standby inverter power supply using an electromechanical relay switching network to connect and disconnect line power between input and output terminals. An overcurrent relay pulser rapidly opens the switching system relay contacts upon transition to battery mode, and a breakover diode network accelerates field collapse in the relay coil attendant to reversion of the relay to its normally closed condition attendant to each battery to line transition. A fault detector establishes acceptable line voltage thresholds, the exceeding of either of which triggers operation from line to battery mode.
U.S. Pat. No. 4,977,351 to Bavaro discloses an emergency lighting system which permits at least one gas discharge lamp to be operated from an AC power source when AC current is present and from a battery when AC signal is not present. To conserve battery power in the DC mode, controls are also provided for turning off the light or reducing its output level in response to such control inputs as an ambient light detector or manually operated dimmer control.
U.S. Pat. No. 5,001,623 to Magid discloses a power supply which automatically adapts to different input power sources. A double pole, double throw, latching relay switches the primaries of the rectifier transformer to a series or to a parallel configuration depending upon the output voltage of the rectifier, thus adapting the circuit to either a nominal 220 V AC source or a nominal 120 V AC source. When a 24 V DC source is connected to the DC input, it is also connected to the rectifier output through a diode, so that power is furnished to the load by whichever source has the higher voltage. In this configuration, a battery pack can be connected to the DC input to supply automatic backup power.
U.S. Pat. No. 5,010,469 to Bobry discloses an uninterruptible power supply having an inverter that operates in either high voltage DC supplied by a line-powered power supply or on low voltage DC batteries. With line power, the inverter operates as an H-bridge inverter. With battery power, the battery voltage is applied to a smaller portion of the inverter's transformer primary. The changeover from line to battery power is determined by when an intermediate voltage tapped from the transformer drops below the battery voltage.
U.S. Pat. No. 5,017,800 to Divan discloses an apparatus for AC to DC to AC power conversion. It includes a rectifier bridge connected to an AC source, the rectifier bridge providing a DC output to DC bus lines. A full bridge of active switching devices is connected across the DC bus lines. The output of the bridge can provide AC output power to the load through a transformer. By providing an external DC power source such as a battery, uninterrupted power can be supplied to the load when the AC power source fails.
None of the above-mentioned references attempts to minimize inverter power consumption when no appliance is connected to the load terminals.